1. Field of the Invention
This invention relates to an optical module which optically couples an optical semiconductor device with an optical fiber by a lens, and a method of producing the optical module. Such an optical module is useful as, for example, a light emitting device or a light receiving device in an optical LAN or the like, or a light source in a bar code reader.
2. Background
An optical module is an optical part in which various optical semiconductor devices are incorporated, and includes an optical connector, an optical collimator, etc. Such an optical module is used in various fields. For example, an optical connector used in the field of optical communication is an optical module which optically couples a semiconductor light emitting device or a semiconductor light receiving device with an optical fiber. In a computer system in which data communication is conducted by an optical LAN or the like, a module of a semiconductor light emitting device and that of a semiconductor light receiving device are paired with each other on a circuit board. Specifically, such a module includes an optical semiconductor device (for example, a semiconductor light emitting device such as a laser diode, or a semiconductor light receiving device such as a photodiode), a lens, and a receptacle core for fittingly holding a ferule of a counter optical plug. The module has a structure in which, when the optical plug is connected, the optical semiconductor device is optically coupled with an optical fiber in the ferule by the lens. An optical collimator which serves as a light source for a bar code reader has a function of converging light emitted from the laser diode by means of a lens so that a specified beam size is obtained at a position separated by a predetermined distance.
In an optical module into which a semiconductor light emitting device is incorporated, when an optical fiber is pulled out from the optical module during operation, light emitted from the semiconductor light emitting device is emitted to the outside as it is. For a laser apparatus, in order to ensure the safety of the human body, a laser safety standard has been established in accordance with the degree of danger with respect to the light amount. An optical module is desired to satisfy class 1. In "class 1," under any conditions, the light amount does not exceed the MPE (Maximum Permissible Exposure) for the eye, safety is ensured in design, and special management is not required.
In order to satisfy the laser safety standard of class 1, an electronic shutter mechanism which is called an open fiber control has been developed. This mechanism operates so that, when an optical fiber is pulled out from an optical module, the driving of a semiconductor light emitting device is electronically stopped. Furthermore, a mechanical shutter mechanism has been developed in which, when an optical fiber is pulled out from an optical module, light emitted from a semiconductor light emitting device is prevented from leaking out of the optical module (see Unexamined Japanese Patent Publication No. Hei. 3-132708).
When the laser output is greater than a certain level, the additional disposition of a shutter mechanism of any kind which satisfies the laser safety standard is necessary and effective. By contrast, in the case of a laser of a low output power, for example, the use of an electronic shutter mechanism requires a larger number of electronic parts (such as ICs), and that of a mechanical shutter mechanism requires a larger number of parts and makes the assembling process complicate. Both the uses are disadvantageous also in the view point of cost. In the event that trouble occurs in such a shutter mechanism, it is impossible to satisfy the safety standard. Therefore, there arises a problem in that, as the structure is more simplified, the reliability is more impaired.
The method which is simplest and has high reliability is to suppress the amount of light emitted from a laser to a range where the safety standard of class 1 for laser apparatuses is satisfied. When the light amount of the output of the laser is merely reduced, however, the properties of the optical module may be lowered.
In an optical module of this kind, conventionally, a configuration is employed in which a ferule stopper is disposed in a receptacle core so that the position of the ferule in the optical axis is positioned in the connecting process (for example, see Unexamined Japanese Patent Publication No. Hei. 4-181904, etc.). A through hole which is small as compared with a lens diameter is formed in the ferule stopper so that a light beam can pass therethrough. Principally, undesired light can be eliminated by sufficiently reducing the diameter of the through hole positioned in the vicinity of the end face of the fiber. In order to establish such a configuration, however, a fine hole must be correctly opened because the through hole is formed at a position where the coupled light beam has a very small diameter (about 100 .mu.m.phi.). In order to correct a positional displacement in a direction perpendicular to the optical axis, moreover, centering must be conducted. Consequently, an optical module having such a structure cannot be produced economically and efficiently.
On the other hand, a spherical lens or a rod lens is generally used as a lens for an optical module. It is a matter of course that a lens of another kind may be used. Among lenses of various kinds, a spherical lens is widely used because of its advantages that a highly accurate product can be easily obtained only by mechanical processing and hence the cost is low, and that the lens has no directionality and hence it is not required to adjust the direction when the lens is to be mounted in an optical module, thereby facilitating the assembly work. Known methods of fixing a lens to a holder include welding using low-melting glass, adhesion using a resin adhesive, soldering, and a mechanical fixation in which an annular elastic member is fitted. Among these methods, the adhesion method has a drawback that a liquid adhesive must be poured into a narrow region and it is difficult to handle such an adhesive before hardening. When soldering is to be performed, a lens must be metalized. Particularly, it is difficult to metalize a spherical lens. Furthermore, metalization causes the lens to have directionality, and hence the mounting work is complicated. The mechanical press fitting and fixation requires additional parts such as an annular elastic member, and hence has drawbacks that the assembly work is complicated, and that the cost is increased. Because of these reasons, the welding method using low-melting glass is advantageous and widely used.
Specifically, an annular low-melting glass compact obtained by press-molding powder of low-melting glass (having a melting point of, for example, about 365.degree. C.) into an annular shape is used. The low-melting glass compact is placed on a step portion of an inner wall of a holder which supports a lens. The assembly is placed in an oven and subjected to a heat treatment at about 400.degree. C. The low-melting glass compact melts, and the lens 50 is welded to the holder 52 as shown in FIG. 1. The glass pool formed as a result of solidification of the melting low-melting glass compact is designated by 54.
When a part in which a device is placed in a hermetically sealed package is used as an optical semiconductor device, a holder which holds a lens is not required to be hermetically sealed. Therefore, the outside air containing moisture easily enters the interior of the holder. As described above, the annular low-melting glass compact used in the glass welding is obtained by press-molding powder of low-melting glass. During the process of dropping the compact to a predetermined position, therefore, fine pieces or powder may be scattered and adhere to the surface of the lens. When, in order to fix the lens, heat melting processing is conducted without taking a countermeasure, the adhering pieces or powder melt and the low-melting glass itself penetrates, whereby films of low-melting glass are locally formed on the surface of the lens. Particularly, low-melting glass is easily affected by moisture. With the passage of time, devitrification (cloudiness) is often produced. As a result, as shown in FIG. 1, devitrified portions 56 caused by films of low-melting glass are formed. The formation of such devitrified portions results in a reduced light amount of the optical module.
Moreover, the outside air containing moisture easily reaches the portion of the glass pool (low-melting glass) inside the holder. The moisture causes a phenomenon that fine cracks are formed in the surface of the glass pool and the glass becomes brittle. This phenomenon reduces the strength of the fixation of the lens. In an extreme case, the lens may drop off.
In order to solve these problems, it may be contemplated to employ several methods. In order to prevent low-melting glass from being devitrified, fine powder or pieces adhering to the surface of the lens may be removed away before heat melting processing. As a matter of fact, however, a work of completely removing away such fine powder or pieces of low-melting glass is very difficult to do. Even if such removal is realized, low-melting glass inevitably penetrates during heat melting processing, and it is impossible to prevent the formation of cracks in the surface of the glass pool from occurring. The interior of the holder which holds the lens may be hermetically sealed. However, the cost is increased and the structure is complicated. As another countermeasure, glass which is relatively hardly devitrified may be used. However, glass having such properties and excellent moisture resistance has a high melting point. Therefore, it is difficult to use such glass as a welding material for a lens.
In order to comply with the use of an optical module or properties required of an optical module, occasionally, an optical filter film of any kind is formed on the surface of a lens which is to be incorporated into the optical module. For example, such a film includes an ND (Neutral Density) filter film for adjusting the light transmittance, and an antireflection coat film. Generally, these films are formed by the physical vapor deposition method such as the vacuum deposition method.
In the case of using a spherical lens, for example, when an optical filter film of any kind is to be formed on the surface of the spherical lens, the spherical lens must be holed by lens holding means such as a fixture. Specifically, the film growth is conducted in the following manner. A number of spherical lenses are arranged on and fixed to a fixture, and the assembly is placed in a film growth chamber for vacuum deposition. Therefore, the film growth is not naturally conducted on a part of the surface of the spherical lens (the portion gripped by the fixture). As a result, although a spherical lens is used, directionality is produced depending on the existence and nonexistence of the optical filter film. When the spherical lens is to be mounted in the holder, therefore, the direction is first adjusted so that the optical filter film exists on and in the vicinity of the optical axis, and thereafter the lens is fixed to the holder. However, the work of fixing the lens to the holder with detecting the distribution of the thin film on the sphere is very cumbersome and hence the assembly work is poor in efficiency. In other words, directionality is produced in a spherical lens which is originally free from directionality, by the film growth, and this directionality largely impedes the lens mounting work.
In order to comply with the above, a countermeasure may be taken in the following manner. A spherical lens is cut into a column-like shape and an optical filter film is formed on remaining spherical portions, or an optical filter film is formed on the surface of a spherical lens and thereafter portions including the portion gripped by the fixture are cut away, thereby obtaining a structure in which a substantially column-like shape is attained, a spherical face remains on the end faces, and the optical filter film is formed on the spherical faces. The obtained lens is dropped into a holder with using the columnar face. According to this countermeasure, the improved shape facilitates the direction adjustment, but the cutting work must be performed on each lens, with the result that the lens is expensive.